Positron emission tomography (PET), optionally in combination with computed tomography (PET/CT), allows besides medical diagnostic imaging the quantitative determination of various parameters. Preconditions for such quantitative measurements on patients are the strict adherence to standardized clinical protocols, as well as an absolute calibration or cross calibration of the tomographs used.
The requirements for uniform clinical protocols as well as the requirements for cross calibration of the tomographs and for methods to determine quantitative values currently are intensively discussed particularly in the context of multi-centre medical studies [WAHL09], [BOEL09], [BOEL09b], [DELB06], [SHYA08] [SCHE09].
Quantitative measurements acquired by tomographs, however, show a strong and unavoidable dependence on the imaging properties of the particular tomograph, notably on the spatial transfer function (Point Spread Function, PSF), which differs from each other tomograph. Due to the different transfer functions, any quantitative assessment is made difficult even for otherwise calibrated tomographs or other certified medical imaging device. This applies particularly to multicenter medical studies which demand a quantitative comparison of the measured data of the participating centers.
In positron emission tomography the most commonly used quantitative measure for the activity measured in a tissue is the “standardized uptake value” (SUV) in different variants [BOEL09]. It has been shown that determination of an SUV can be performed with great repeatability when a strictly defined examination protocol is adhered to and the same tomograph is used in each measurement.
It is these measurements, however, which possibly show very different results, if different tomographs are used, even if the devices are calibrated. The differences mainly are due to the partial volume effect (PVE), which causes the activity of small objects to be systematically underestimated. This effect is closely related to the spatial transfer function of the particular tomograph.
Aside from the spatial resolution determined by the half width of the transfer function the exact shape of the transfer function is of particular interest. Thus, even when having same spatial resolution, different tomographs may show very different partial volume effects (PVE). Various methods to minimize the impact of PVE on quantitative measurements, especially SUV measurements, have been proposed [KEYE95] [STRA91] [BOEL03] [WEST06] [SORE07].
Using these often complex methods and if need be employing independent measurements for the determination of the volume of an object, it is possible to increase the accuracy of the measurements or to extend the available measuring range to towards smaller objects. Nevertheless the results still depend on the tomograph used.
Therefore, in the context of multi-centre studies, the benefits of those methods are very limited.
In quantitative PET studies and particularly in studies, which need to be carried out repeatedly over a long period of time or at different sites, a standardized tomograph would be required, which could provide reproducible measurements anywhere at any time.
Unfortunately, such a standardized PET system is not available, on the one hand due to the large number of available systems of different manufacturers, on the other hand as a result of continuous technical development of the systems and the resultant constant changes of devices.